You can add a related list to the contacts in Salesforce to have a handy way of showing who signed a document.
- Go to contacts, select a contact
- Click on "Edit Layout" (right upper corner)
- In the layout choose "Related lists" and then click on "SignRequest Signers"
Note: As indicated in step 1, this only works when the contact is imported from Salesforce, not when an email is entered in the SignRequest pop-up form.